Generally, an optical power splitter which is called a splitter or an optical divider which has a function of transmitting one lightwave to a plurality of subscribers (1×N) is a very important device in a broadband optical communication network (the optical power splitter of 2×N can also be used according to a configuration method of a communication network), and is used in data communication, a passive optical network (PON), fiber-to-the-home (FTTH), and so on.
Important features required in the optical power splitter are 1) excellent branching uniformity (with low variability with respect to an input optical wavelength, polarization, or temperature change), 2) low light loss, and 3) low-cost mass-producibility. However, among current technology of manufacturing the optical power splitter, planar lightwave circuit (PLC) technology has been known as the best technology of implementing the features, and the PLC technology is currently the main method used to manufacture optical power splitters.
The method is used to configure an optical circuit in which a core and a clad surrounding the core are formed by depositing multiple layers of silica or a thin polymer film while performing photolithography and etching on a silicon or quartz substrate and the optical signal is divided and mixed according to a shape of the core using a difference in a refractive index between the core and the clad.
There are various methods of configuring the optical power splitter since various structures in which one lightwave divides into a plurality of lightwaves have been proposed. The structures are largely classified as a star shape and a tree shape depending on a shape of dividing optical power, and methods of using a Y-branch lightwave circuit, a multi mode interferometer (MMI), a star coupler, a directional coupler, and so on have been offered.
However, in devices using a structure such as the MMI, the star coupler, the directional coupler, and so on, a split ratio of the optical power with respect to an incident optical wavelength or polarization change is varied. Accordingly, the structures are not used for commercial products in a market of a passive optical network (PON), etc., but are used for special products.
On the other hand, since a Y-branch optical power splitter has low wavelength dependence and enables miniaturization, the Y-branch optical power splitter is included in most commercial optical power splitters as a fundamental component. However, due to a structure in which a unit splitter of a Y-branch (1×2) is serially connected in a tree shape, a change amount of a split ratio depending on branching uniformity of output ports, manufacturing processes, temperature, wavelength, polarization, etc. increases in proportion to the connection number of trees. Accordingly, the split ratio of the unit splitter should be very exact and the change amount of the split ratio should be small.
When methods of manufacturing PLCs are classified according to a method of manufacturing a lightwave circuit core, there are 1) an embossed process method of forming an embossed core on a plane clad substrate and covering a lid clad thereon (this is currently the main method used to manufacture PLCs), and 2) an engraved process method of forming an engraved groove with a core shape, filling a core therein, planarizing the core by chemical mechanical polishing (CMP) or an etch-back process, and overlaying the lid clad.
Meanwhile, input and output cross sections of a manufactured PLC chip are polished as an inclined plane (an angle of about 8 degrees), and a polished optical fiber array block which is also polished as the inclined plane is bonded to the polished cross sections with epoxy. Here, the optical fiber array block is a block in which an optical fiber array is fixed between optical fiber fixed groove blocks including upper and lower parts. Each of the upper and lower parts has a thickness of about 1 millimeter, and the optical fiber array block has a total thickness of 2 millimeters.
Accordingly, the optical fiber array block and a PLC processed on a substrate with a thickness of about 1 millimeter have a difference in a bonding area. In order to remove distortion of epoxy bonding, a lid glass with a thickness of about 1 millimeter is added on the upper side of the substrate of the PLC. However, since the epoxy is largely different from the PLC in material characteristics, distortion or detachment may occur due to changes in temperature/humidity. Further, when bonding the substrate of the PLC and the lid glass, considerable and additional manufacturing efforts such as thickness control of the epoxy with respect to the entire surface of the substrate are required.
FIG. 1 is a diagram illustrating a structure of a Y-branch lightwave circuit according to an embodiment of a conventional art.
Referring to FIG. 1, the Y-branch lightwave circuit according to an embodiment of a conventional art includes an input lightwave circuit 10, and first and second output lightwave circuits 20 and 30.
Here, the input lightwave circuit 10 is configured to input an optical signal through an input side cross section, and have a shape that widens toward an output side cross section.
The first and second output lightwave circuits 20 and 30 are configured to extend symmetrically with respect to a center line from the output side cross section of the input lightwave circuit 10.
The conventional Y-branch lightwave circuit optimizes a width and a length of the input lightwave circuit 10, and a branch angle of the first and second output lightwave circuits 20 and 30, and light loss is minimized when an interval between the first and second output lightwave circuits 20 and 30 is zero. However, when the lightwave circuit is manufactured, the interval between the first and second output lightwave circuits 20 and 30 which can be implemented in a photo exposure process has a minimum of 1 to 2 microns. Accordingly, certain light loss may be incurred.
Meanwhile, in the conventional embossed process method, even if the interval between the first and second output lightwave circuits 20 and 30 is minimized at a branch point, since an etching depth in a narrow gap between the first and second output lightwave circuits 20 and 30 is smaller than that of other areas due to an RIE-lag phenomenon in an etching process of a lightwave circuit core, lower portions of cross sections of the two first and second output lightwave circuits 20 and 30 are connected, and bubbles in the narrow gap are generated in a lid clad process, or the core is inclined since the two first and second output lightwave circuits 20 and 30 pull each other due to melting and sintering of the lid clad.
Accordingly, in order to solve the problem, oxide additives such as boron oxide (B2O3), phosphorus pentoxide (P2O5) are heavily doped into the lid clad material, and then the core is melted and damaged by the lid clad in the lid clad process and the light loss is increased.
Further, the lid clad process is a process requiring a long time to deposit a very thick film with a thickness of about 20 to 40 microns. Accordingly, productivity is greatly reduced, Further, the lightwave circuit core receives a stress due to a large difference in thermal expansion coefficients between two materials configuring a bottom clad substrate of a quartz glass and a heavily doped thick lid clad, and thus degradation of branching uniformity, particularly, polarization dependent loss (PDL), is increased.
The lid clad process varies in a thickness of a deposited film compared with a conventional semiconductor process with a thickness less than or equal to 1 micron. Accordingly, no commercial apparatus is appropriate for this, much time and effort are needed to deposit the thick film as an optical film with high quality, and thus the lid clad process is the most difficult process in manufacturing PLCs.